In general, a heat-sensitive recording material in which heat energy (Joule heat) from a heat-sensitive head, heat pen, or the like is applied to a basic dye which is colorless or light-colored at room temperature and an organic developer to obtain a color recording is already widely put into practical use.
Examples of the performances required for the heat-sensitive recording material include the whiteness of the unprinted portion, the whiteness of the unprinted portion under various environmental conditions, the coloring density of the printed portion, the storage stability of the printed portion, and the like.
The storage stability of the printed portion means the performance in which loss of the printed image is not caused even in a case where the printed image is placed in a high humidity environment, a case where water is adhered thereto, a case where oil is adhered thereto, a case where a plasticizer is adhered thereto, or the like.
On the other hand, the required performances for prints formed by the heat-sensitive recording material largely depend on the dyes, developers, and sensitizers which are the main components of the heat-sensitive recording material and, in particular, the influence of the developer is large.
For this reason, synthetic compounds derived from petrochemicals such as phenolic compounds or sulfonylurea compounds have been proposed as developers satisfying the required performances described above. Among these, many phenolic compounds have been developed and put to practical use.
However, there are concerns of some phenolic compounds exhibiting endocrine disruption and therefore use thereof is recently being suppressed. For example, bisphenol A (2,2-bis(4-hydroxyphenyl)propane) has been used in large quantities as a raw material for polyester or as a developer for heat-sensitive paper; however, bisphenol A is classified as a Type II and a Type III Monitoring Chemical Substance in the Japanese Act on the Evaluation of Chemical Substances and Regulation of Their Manufacture, etc., before amendment and as a priority assessment chemical substance in the same Act after amendment, furthermore, the use thereof has already been restricted in the EU, the United States, Canada. Japan, and the like due to suspicions of being an endocrine disrupting substance.
Bisphenol S(4,4′-dihydroxydiphenylsulfone) is registered as a designated chemical substance in the Japanese Act on the Evaluation of Chemical Substances and Regulation of Their Manufacture, etc., before amendment due to concerns of chromosomal abnormalities and the like and regulated as a Type II Monitoring Chemical Substance.
As a non-phenolic developer such as a sulfonylurea compound, the use of a synthetic compound such as 4,4′-diaminodiphenylalkane as a raw material has been proposed (Patent Document 1). The use of a non-phenolic developer using an acylated amino acid where the main raw material is a natural amino acid has been proposed (Patent Document 2). In addition, a heat-sensitive recording material which obtains an image by applying a browning reaction (Maillard reaction) by heating amino acids and saccharides has also been proposed (Patent Document 3).